1. Field of the Invention
This invention relates to a microphone and in particular to a novel condensor microphone providing unidirection and nondirectional capabilities.
2. Prior Art
Condensor microphones have a high output impedance and generally an active element such as a field effect transistor has been mounted in the microphone housing. Such microphones generally incorporate a diaphragm mounted in the housing, and a backplate assembly which consists of a backplate and a support for the backplate providing support therefor a predetermined distance from the diaphragm. The housing also encloses a printed circuit board on which is mounted an impedance converting means including an active element such as an FET and resistance elements. Output and power supply leads are supplied to the microphone and connected to the FET and the circuit board. Because of these complexities it was difficult to assemble such microphones when small in size, rendering such microphones expensive and subject to failure due to lead breakage.
The above problems have been solved by Ishibashi et al. as described in U.S. Pat. No. 3,775,572. There, an active element and necessary lead wires are molded in an insulating member, which supports a backplate thereon, one of the leads of the active element being connected to the backplate, whereby wiring and assembling are simplified. However, since active element and backplate are integrated in a body, if the conductive material used for the backplate is not coated on the insulating member uniformly, or if an upper surface of an insulating member is not formed flatly, the distance between the backplate and diaphragm is not uniform throughout. If such a reject backplate assembly is made, it cannot be used for a microphone even though the FET is good. This means that the FET is needlessly thrown away.
Further, there is a need for a microphone which can be switched for nondirectional to unidirectional reception and vice versa. Directional reception switching has been obtained by providing an additional acoustic chamber behind the diaphragm of the microphone. This has required high precision work and the microphone becomes bulky, making it unsuitable for incorporation in small size apparatus.